There are about 160 different grades of oil. The proportions of products from the oil are dependent on the grade of the crude oil. You get more gasoline/diesel/distillates per barrel from a "light, sweet" crude oil (high gravity, low sulfur) than a "heavy, sour" crude. Also, given refinery is built to process a certain range/grade of crude oil and can't efficiently process other grades.

The short answer is, our refineries are set for OPEC-type oil and can't process domestic crude.

Well done Mark
I've worked in the oil industry 40 years, or basically my whole working life, not many outside the industry grasp even basic facts about it, but there again I guess that's no different than most industries.

The Saudis are still trying to do an IPO for 5% of ARAMCO, and it looks like they have been pretty successful in coordinating OPEC to reduce pumping and dry up the glut.

I try to do at least some research on a given subject BEFORE I open my mouth (or keyboard). This one was easy. I already knew the terms light/sweet and heavy/sour and the best grade is light/sweet, but not what those terms meant or why one is preferred over another. Now I know.